September 2001 National Climate Report

North American 500 Mb Maps

The mean upper-air pattern during September was generally characterized by positive height anomalies over portions of the west while height anomalies in the rest of the country were near zero. A persistent area of high pressure brought much above normal temperatures to many western states while troughiness in the eastern half of the nation contributed to a cool September, most notably in the southeast. Temperatures in the northeast were near to above normal. Additional information on hydrometeorological analysis and forecasting can be found at the Hydrometeorological Prediction Center's Web Page. The principles behind the 500 mb flow are briefly explained here.

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National Temperatures

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The graph to the left shows monthly mean temperature averaged across the contiguous United States based on long-term data from the U.S. Historical Climatology Network (USHCN). The 2001 value is estimated from preliminary Climate Division data using the first difference approach. September 2001 ranked as the twenty-ninth warmest September in the 1895 to present record. The preliminary nationally averaged temperature was 66.3° F (19.1° C) which was 0.9° F (0.5° C) above the long-term mean. The September temperature values from 1895 through 2001 are available.

Just three years ago in 1998, the warmest September in the record occurred when the nationally averaged temperature reached 69.5° F (20.8° C).

Lower Tropospheric Temperatures

Data collected by NOAA's TIROS-N polar-orbiting satellites and adjusted for time-dependent biases by NASA and the Global Hydrology and Climate Center at the University of Alabama in Huntsville indicate that temperatures in the lower half of the atmosphere (lowest 8 km) were below average over the U.S. for September 2001. The average lower tropospheric temperature over the continental U.S. was 0.7° F (0.4° C) below the 1979-1998 mean. This marks a change from the previous four years where temperatures were near to much above the mean.

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Temperature Departures

In general, most of the western half of the nation was warmer than average. The largest positive anomalies occurred from Montana southward into Idaho, Nevada and Arizona. Positive anomalies also occurred in many northeastern states. The bulk of the eastern half of the country and coastal areas of California and Washington experienced cooler than normal temperatures. Puerto Rico and Hawaii were near normal while Alaska was near to above normal.

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Maps showing how the temperature anomalies varied day-to-day and how the day-to-day temperature variability compares to the long-term average are available. For further regional analysis, view NCDC's regional page.

Standardized Temperature Anomalies

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Positive standardized temperature anomalies for September 2001 were generally located in the western half of the nation and in portions of the northeast. Negative anomalies were present in most of the eastern half of the country with the largest anomalies occurring in south Texas, the extreme southeast and areas of the mid-Atlantic. The adjacent animated map shows the geographical pattern of temperature anomalies for the last 12 months compared to a base period of 1931-1990. For further regional analysis, view NCDC's regional page.

National Precipitation

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The graph to the left is a time series depicting precipitation averaged across the nation. Based upon preliminary precipitation data, September 2001 was near average, ranking forty-sixth in the 1895 to present record. While nationally averaged precipitation was near the long term mean, there was regional variability. Nebraska and Kansas were notably wet receiving much above normal precipitation for the month. For further regional analysis, view NCDC's regional page.

Precipitation Anomalies

The map to the right, based on approximately 250 airport stations, shows September 2001 total precipitation as a percent of the 1961-1990 station normals. Above normal precipitation generally occurred in the Gulf coast states, around the western Great Lakes, in the central Plains and in the Midwest. The rest of the country was mostly dry including Alaska, Hawaii and Puerto Rico. For further regional analysis, view NCDC's regional page.

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National Temperatures - Warm Season (April-September)

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The graph to the left shows April-September mean temperature averaged across the contiguous United States based on long-term data from the U.S. Historical Climatology Network (USHCN). The value is estimated from preliminary Climate Division data using the first difference approach. April-September 2001 ranked as the fourth warmest such period in the 1895 to present record. The preliminary nationally averaged temperature was 67.4° F (19.7° C) which was 1.6° F (0.9° C) above the long-term mean. The Warm Season temperature values from 1895 through 2001 are available.

This is the fourth consecutive year where April-September temperatures have averaged above the long term mean.

National Precipitation - Warm Season (April-September)

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The graph to the left is a time series depicting precipitation averaged across the nation. Based upon preliminary precipitation data, April-September 2001 was near average, ranking thirty-seventh driest in the 1895 to present record. Twelve of the last thirteen April-September periods were near to above normal. For further regional analysis, view NCDC's regional page.

References

The following NCDC datasets are used for the historical U.S. data: the climate division drought database (TD-9640), and the hurricane datasets (TD-9636 and TD-9697). It should be noted that the climate division drought database consists of monthly data for 344 climate divisions in the contiguous United States. These divisional values are calculated from the 6000+ station Cooperative Observer network.

Citing This Report

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